The diverse biologic effects of retinoic acid (RA) are thought to be mediated through specific RA receptors (RARs). RA induces the granulocytic differentiation of leukemia cells in patients with acute promyelocytic leukemia (APL). Most RA-sensitive APL samples harbor a specific 15:17 chromosome translocation, which involves the RAR-alpha locus and leads to an aberrant RAR-alpha fusion transcript (designated PML-RAR-alpha). The pathophysiologic consequences of the generation of this aberrant PML-RAR-alpha fusion transcript and its relationship to leukemogenesis are presently unclear. It is also unknown what role, if any, RA might play in regulating normal hematopoiesis. This proposal plans to approach these questions by utilizing retroviral-mediated gene transduction to transfer aberrant RA receptor constructs into hematopoietic cell lines and normal hematopoietic stem cells. The specific questions to be addressed include the following: Specific Aim I) How does the expression of the aberrant PML-RAR-alpha fusion gene relate to the pathogenesis of APL? To explore the pathophysiologic consequences of the generation of the PML-RAR-alpha fusion gene, we will utilize retrovirus-mediated gene transduction to express this fusion gene in both RA-sensitive and RA- resistant hemopoietic cell lines and compare the differentiation of the uninfected vs. retrovirus-infected cells in response to RA. We will also infect murine hemopoietic stem cells with a retroviral vector harboring the PML-RAR-alpha fusion gene to study the leukemogenic potential of this aberrant RA receptor in vivo. Specific Aim II) What roles do RA and RA receptors (RAR-alpha) play in regulating normal hematopoiesis? Several lines of indirect evidence suggest that RA and the RA receptor (RAR-alpha) may be involved in regulating normal hematopoiesis. To directly approach this question we will determine the effect on hematopoietic differentiation of introducing a mutant "dominant negative" RAR-alpha into a hematopoietic cell line that mimics several aspects of normal hematopoiesis as well as into normal murine hematopoietic stem cells in vivo. Although our proposed studies are confined to hematopoietic cells, they may eventually provide insight into the role normal or abnormal RA receptors might play in the development of other human malignancies.